Workpieces are supported on support racks and carried along by a conveyor through a finishing system which may include the steps of cleaning, rinsing, drying, coating and baking. Usually finishing includes travel through an electrostatic spray booth wherein the workpieces are sprayed and coated with electrically charged particulate. Workers then remove the finished, coated workpieces from the racks and reuse the racks for subsequent operations. Since the racks are constantly recycled through the finishing system, they become encrusted with multiple layers of the coating material after several runs.
It is important in electrostatic coating operations that the support rack be electrically conductive, whereby workpieces can be maintained in a grounded electrical state. The workpieces are electrically connected to ground potential through the conductive support rack so that the electrically charged coating particulate is attracted by an electric field to the workpieces. Good grounding is important in order to provide a uniform particulate distribution over the entire workpiece. Thus, it is desirable to provide a support rack wherein the electrical contact between each workpiece and the main frame of the support rack is maintained as a good, low resistance electrical contact so that the suspended workpieces will remain well grounded.
It is known to provide support racks having support hooks upon which articles are hung. Each painting or coating operation applies a coating layer to the exposed, article-engaging portion of the support hooks which coating entirely covers the hooks except for the small area of contact with the workpiece. Unless a very similar workpiece is to be run in a subsequent operation, whereby the workpiece would be in electrical contact with the hook at an identical part, there will not be good electrical contact between the support hook and the workpiece. Thus, where a different workpiece is to be transported in a subsequent run it is necessary to provide a clean support hook to assure the requisite good electrical contact between the workpiece and the support hook. Accordingly, it is desirable that the support hooks be detachably engaged to the main frame of the rack so that the hooks can be periodically removed and replaced with new or cleaned hooks. Simple and rapid interchangeability of the support hooks is also desirable so that different sizes and shapes of support hooks can be interchanged as desired to allow for customization of the support rack to suit the requirements of a wide variety of runs. It is desirable to provide this interchangeability while still maintaining good electrical contact between each support hook and its respective support rack.
One workpiece support rack which is shown in the prior art is described in U.S. Pat. No. 4,217,853, and another is shown in German Pat. No. 1,079,916. A problem with the prior art racks is that they are either too expensive to manufacture or else do not provide the desired rearrangeability. It is desirable to provide a support rack which lends itself to simple and inexpensive manufacture while still providing for flexibility and also good electrical contact of the workpieces supported.